1. Technical Field
The present invention relates to a thin-film-transistor (TFT) backplane for a display device, and more particularly, to a TFT backplane for more power efficient operation of the display and a method of fabricating such a TFT backplane.
2. Description of the Related Art
Flat panel displays (FPDs) are employed in various electronic devices such as mobile phones, tablets, notebook computers as well as televisions and monitors. Examples of the FPD includes a liquid crystal display (LCD), a plasma display panel (PDP), an organic light emitting diode (OLED) display as well as an electrophoretic display (EPD).
Generally, pixels in a FPD are arranged in a matrix form, and generate light (luminescence) upon electrical activation from an array of thin-film-transistors (TFTs), also known as TFT backplane. A TFT backplane is an important part of a FPD as it functions as a series of switches to control the current flowing to each individual pixel. Until recently, there have been two primary types of TFT backplane technologies, one using TFTs with amorphous silicon (a-Si) active layer and the other using TFTs with polycrystalline silicon (poly-Si) active layer.
A TFT with a-Si active layer generally has lower carrier mobility (μ) than that of a TFT with poly-Si active layer. Thus, making a high speed drive circuit (e.g., pixel circuit, gate drive integrated circuit, data drive integrated circuit) for a display is difficult with the TFT backplane employing a-Si TFTs.
A layer of amorphous silicon can be subjected to a heat-treatment using a laser beam to form polycrystalline silicon active layer. The material from this process is generally referred to as low-temperature polycrystalline silicon (LTPS). In general, the carrier mobility (μ) of LTPS TFTs is higher than the a-Si TFTs by as much as 100 times (>100 cm2/V·s). Despite the higher carrier mobility, LTPS TFTs of a backplane tend to have variations in their threshold voltages (Vth) due to the existence of grain boundaries. Such non-uniform threshold voltages among the TFTs employed in a TFT backplane result in display non-uniformity referred to as the “mura.” For this reason, a display drive circuit implemented with LTPS TFTs often requires a complex compensation circuit, which in turn, increases the manufacturing time and cost of the display.
For flexible displays, a-Si TFTs or LTPS TFTs the backplane must be formed at temperatures sufficiently low to prevent thin plastic or glass substrates from degrading. However, lowering the temperature during the fabrication process degrades the performance of the TFTs, limiting their use for flexible displays.
Such disadvantages of the silicon based TFTs called for yet another type of backplane technology, which employees TFTs having their active layer formed of a metal oxide material. In particular, oxide TFTs offer an attractive alternative to silicon based TFTs because of their high mobility (>10 cm2/V·s) and low process temperature (<250° C.), compared to those of a-Si TFTs. The lower leakage current and the scalability to any glass size make the oxide TFT a promising candidate for making a high performance TFT backplane for displays at low cost.
Stable and high-yield production of a TFT backplane employing oxide TFTs requires optimization of the TFT design, dielectric and passivation materials, oxide film deposition uniformity, annealing conditions, and more. Also, manufacturing process variations make it difficult to tightly control the operating characteristics of such TFTs, including their threshold voltages. For example, adopting the etch-stopper type oxide TFTs can improve the reliability, but such a design suffers from high parasitic capacitance and complicates the manufacturing process. Further, the etch-stopper type limits how short the TFT channel can be, thereby affecting the overall size of the display backplane or the aperture ratio in the pixels of the display. As such, the task of designing the driving circuitry for a display becomes complex in view of such constraints.